The present invention is a method which enables leaks, both known and unknown, from containers such as storage reservoirs, particularly geological reservoirs and formations, to be automatically diagnosed and sealed without the need for external intervention.
Therefore the present invention can be applied to diagnose and seal carbon dioxide (CO2) leakage from geological reservoirs, including both hydrocarbon reservoirs and aquifers, and is therefore of particular applicability to the environmental, petroleum engineering and CO2 storage fields.
The leakage of a fluid from containers is of major concern in many industries on account of the economic losses, environmental damage and hazard to health and safety that can result. Where storage of large quantities of a hazardous fluid is contemplated the potential loss or harm caused is increased.
It is therefore desirable to have methods for detecting leakage and for sealing any leaks that may occur. This is particularly the case when the container is difficult to access for inspection or is simply too large for constant or even periodic monitoring, for example an underground geological formation.
CO2 capture and sequestration (storage) by various techniques is proposed as a means of reducing the quantity of this gas in the atmosphere, in order to combat global warming. Forming a major part of current CO2 mitigation strategies is the safe storage of large volumes of CO2 over long periods through its injection into and storage in subsurface reservoirs, or “geological sequestration”.
Appropriate geologic targets for CO2 sequestration are generally brine-bearing formations (those containing nonpotable waters), and mature or abandoned oil and gas reservoirs.
However there are major concerns with respect to the integrity of seal in these geological subsurface reservoirs, and consequently the potential leakage of injected CO2 from the reservoir into which it is injected, into neighbouring permeable formations, or into the atmosphere.
Such leakage is a concern for many reasons, but chiefly because it may contaminate existing energy, mineral, and/or groundwater resources, pose a hazard at the ground surface, and will contribute to increased concentrations of CO2 in the atmosphere.
The diagnosis and sealing of storage reservoir leakage, particularly CO2 leakage from geological reservoirs, is of major concern to many industries and governments and also to the public.
Even small but continued leakage is undesirable, as it would result in return of the gas to the atmosphere, defeating the object of storage. Similar considerations apply where bulk quantities of other fluids are stored.
The two main potential leakage pathways for geologically stored CO2 are, first, leakage of injected CO2 through natural pathways such as faults and fractures (known and unknown) in a geological formation, and second, leakage through improperly cemented existing or abandoned wells.
In practice, identifying these potential leakage locations and executing effective remedies presents a huge problem for those engaged in geological sequestration and storage of CO2.
In geological sequestration of CO2, caprock can provide a low permeability and capillary barrier that can prevent upwards migration of CO2. In some cases, however, it is possible that there are undetected faults, fractures or sand streaks within the caprock, or that the CO2 spreads outside the intended storage formation and thus round the caprock. If the CO2 plume encounters a permeable fault or fracture, or permeates round the caprock, it may leak towards the surface.
Another potential leakage scenario applies to wells, typically oil or gas wells, both existing and abandoned. Possible leakage pathways within an existing well can include preferential flow pathways along the rock-cement interface, along the casing-cement interface, and through degraded materials.
Also, while intact well cements have very low permeability (of the order of 10-20 m2/10−5 milliDarcy), and hence are good materials to use in well completions, the overall permeability of well materials as a whole is very sensitive to relatively small changes in its configuration.
For example, a thin, say 1 millimeter, degraded zone of cement, with very large permeability in the degraded zone, or an annulus associated with poor bonding of cement to rock or casing, can lead to large effective permeability if the annular opening is continuous along the well.
The extreme sensitivity of permeability values to small-scale irregularities, coupled with the large number of wells that typically exist in mature hydrocarbon reservoirs, gives a high leakage potential from well-based CO2 sequestration.
Four basic approaches have been suggested for stopping CO2 leakage from reservoirs:    (1) Reducing the pressure in the storage reservoir. However, this is not a straightforward task, and would involve the removal of CO2 from the reservoir, consequently creating CO2 disposal challenges, and negating the original aims of geological sequestration.    (2) Interception and extraction of the CO2 plume from the reservoir before it leaks out of the storage structure. However, this requires accurate information on the location of the plume, and also the use of techniques and facilities to extract the plume. Again disposal challenges are faced, negating the original aims of geological sequestration.    (3) In case of CO2 leakage into another rock formation, inducing greater pressure in the rock formation into which leakage is occurring. However, this is a complicated exercise, as it requires information on the specific target area, amount and type of fluid and an accurate monitoring program.    (4) In case of leakages with known and accessible locations, for example within wells, plugging the leak with low permeability materials, for example cement, thereby stopping leakage out of the storage formation. However, this also requires accurate information on the type and amount of material to be used, as well as on the specific target area.
As all of the current leak prevention methods described above require, in the first place, information on the type, extent and location of the leak prior to implementing any of the corresponding remedial work, an analysis of potential leakage of injected CO2 from a formation, as well as an evaluation of the feasibility of the proposed prevention method, is generally required.
In this analysis the preferential flow paths for the leakage of injected CO2 through natural pathways, such as faults and fractures, and/or through improperly abandoned or cemented wells should be identified. A realistic geological picture may be required. This comes with a great degree of uncertainty, making the task of identifying the leak target area very difficult, if not impossible. An accurate and extensive monitoring program may also be required to detect the existing preferential leakage flow path and/or detect the extent of degrading. Both of these items require sophisticated and specifically designed equipment, set-ups and procedures. There is also a large delay time from between when a leak occurs, when it is detected, and the point by which a sealing mechanism (if at all possible) can be put in place once the target area has been located accurately enough.
The present invention includes a unique method for effectively stopping CO2 leakage pathways from geological storages when and where they happen.